1. Field of the Invention
The present invention relates to a system for controlling the height of a vehicle.
2. Description of the Prior Art
Japanese Utility Model Application Public Disclosure No. 60-102114 (1985) discloses one example of a conventional vehicle height control system. The disclosed vehicle height control system primarily comprises a pump for generating oil pressure, a charge-discharge valve which is interposed between the pump and a cylinder provided for each wheel, three accumulators (gas springs) which are connected in parallel through a passage to a communicating passage connecting together the cylinder and the charge-discharge valve, and a solenoid-operated directional control valve which is provided in the intermediate portion of the passage connecting the accumulators, the solenoid-operated directional control valve being changeable between two positions, that is, a conducting position where the valve produces no orifice resistance, and a throttling position where the valve provides a proper orifice resistance.
The conventional vehicle height control system is arranged such that oil pressure that is generated by the pump is charged into and discharged from the cylinder through the charge-discharge valve to effect vehicle height control, while the solenoid-operated directional control valve is properly opened and closed to bring the gas springs in and out of communication with each other, thereby changing the overall spring constant of the gas springs provided for the cylinder, and thus making it possible to control the vehicle attitude stabilizing characteristics and provide a comfortable vehicle ride.
Incidentally, it is desired that the vehicle height control system should be as small in size as possible, because it is provided in a limited space between a vehicle body and an axle. However, the above-described conventional vehicle height control system includes a solenoid-operated directional control valve which is arranged to be changeable between two positions, that is, a conducting position where the valve produces no orifice resistance, and a throttling position where the valve provides a suitable orifice resistance. As such, it is necessary to ensure a sufficiently large orifice area to establish a conducting state which is sufficiently unrestricted in contrast to the case of the throttling state. If such a large orifice area is ensured, a correspondingly large pressure difference is produced in the throttling state, so that a large-sized solenoid must be employed for the directional control valve in order to allow the valve to be opened and closed reliably against such a large pressure difference, thus resulting in an increase in the overall size of the system.